I'm pretty sure they got this 'original' idea from the practice of their Muslim contemporaries. This is a very similar concept to the origin of cheque where it's too dangerous to carry gold and cash along the silk road from Middle East countries to China and beyond.
Stupid question, but wouldn't stealing a cheque work back then? I mean I can imagine only known people would be allowed to cash it. Plus there's more benefits to carrying a cheque, large quantities get difficult to move around (same with cash money, which is one reason that higher denominations are discouraged)
Worth bearing in mind the type of people using this service were landed gentry. A Turkish bandit wouldn't do well turning up at a Templar fortress trying to redeem some French Lordlings's gold but you still have to expect unscrupulous Europeans would be stealing from each other.
I'd expect the KYC was pretty strong at the issuers that handled gold/property. Your family would be known, not just the individual. I think they were redeeming food/lodgings/services while on route so defrauding a remote fortresses might not be at all rewarding. To score big I think you'd need to present as landed gentry at places like London which sounds hard... your target would need to have no friends/relations... but it's the premise of 1000s of historical fiction novels so eh?!
Wax seals and such were the authentication tools at the time. It might hobble attempted fakes but I would assume thieves would steal the cygnet rings and such too. I've seen claims the Templars used cyphers but that type of history gets blurred with the romantic stories. Encoding a password onto the cheque seems painfully obvious to us today.
I imagine there was a chain of custody of the person themselves. Templars were protecting the pilgrims not just their gold so they accompanied them from location to location so the chance of someone unknown turning up at a gate with cheques to cash is less likely. I also imagine they were in groups with others of similar stature with similar arrangements so the difference between an authentic wealthy traveller and a chancer might be quite vast. I imagine there was a network of letters between Templar sites tracking the notable Lords so faking your way into someone else's identity would be quite tricky.
From what I gather, it foremost may simply have been a risk based business where the vast profit covered the losses.